1. Field of the Invention
The invention relates to an apparatus for extracting an irradiation field from a radiation image.
2. Related Background Art
Owing to the progress of a recent digital technique, a radiation image is converted into a digital image signal, the digital image signal is image processed, and a resultant image is displayed on a CRT or the like or printed and outputted. Upon photographing of a radiation image, to suppress an influence on a region out of a necessary field by an X-ray, prevent scattering from the region out of the necessary field, and prevent a reduction of contrast, an irradiation field narrow-down for allowing a radiation to be irradiated only to the necessary field is generally performed.
To perform the image process, processing parameters are determined from a distribution of density values and the image process is executed on the basis of the determined parameters. When the irradiation field narrow-down is performed, if all of input data is referred to at the time of deciding the processing parameters, information out of the necessary field, that is, unnecessary information is used for determining the image processing parameters. A problem such that the optimized image process cannot be performed to the necessary field occurs.
It is, therefore, necessary to extract the irradiation field and decide the image processing parameters from the information of only an interesting field. For example, the following methods have been proposed as a method of extracting such an irradiation field.    (1) Characteristic points having edge components are detected by a difference between pixel values of two adjacent pixels in an image and an irradiation field is extracted on the basis of the detected characteristic points.    (2) An image field is divided into small regions and an irradiation field is extracted on the basis of density distribution values in the small regions.
According to the method of (1), since the characteristic points are detected by the difference between the pixel values of two adjacent pixels in the image, they are easily influenced by noises or an X-ray shielded object and a possibility that wrong characteristic points are detected is high. Although a method of performing a pre-process by using a smoothing filter or the like in order to reduce the influence of the noises is also considered, according to a smoothed image, there is a problem such that it is difficult to distinguish an edge based on a true irradiation field edge and an edge that is falsely generated due to a scattering line.
According to the method of (2), even in the interesting field, for example, in a lung field edge portion, the density value suddenly changes and there can be a case where a change ratio of the density is higher than that of the irradiation field edge portion. Particularly, in the field where the lung field edge portion, the ribs, and the lung field are come into contact, a distribution value is large. Consequently, there is also a problem such that candidate points of the irradiation field are extracted also from a region out of the irradiation field edge portion and it is difficult to distinguish them.